"If the truth gives pain, it is not the fault of the teacher, nor of the reader who hears it for the first time, but of error, which stabs and stings before it will surrender its victim." M.M. Mangasarian

When an insurance firm boss saw a field hospital for the poor in Virginia, he knew he had to speak out. Here, he tells Paul Harris of his fears for Obama's bid to bring about radical change

Patients without health insurance get dental care at a free clinic in Wise, Virginia, held every July for the past three years. More than 25,000 were treated in a weekend Photograph: John Moore/Getty Images

Wendell Potter can remember exactly when he took the first steps on his journey to becoming a whistleblower and turning against one of the most powerful industries in America.

It was July 2007 and Potter, a senior executive at giant US healthcare firm Cigna, was visiting relatives in the poverty-ridden mountain districts of northeast Tennessee. He saw an advert in a local paper for a touring free medical clinic at a fairground just across the state border in Wise County, Virginia.